Hilarious (except for the money part), I just had this same problem. I pressure washed the engine. All the plug wire seals held accept cylinder number 1. A bit of water seaped past the rubber seal and the car would misfire intermittently, especially while under load. The misfiring did not start until at least a few weeks after the wash so I didn't put the two together immediately. Assuming that it was going to be something more difficult to diagnose I waited a few months before even attempting to diagnose the problem. It took all of two minutes into the diagnosis to discover that there was still water in the number 1 plug hole. It made a bit of a mess of the top of the plug but no permanent damage (except to my ego.) Cleaned things up and all was well.
Brett